An orifice method of producing a high velocity stream

Abstract

Earlier papers (Binnie 1938 and 1940) have dealt with the uniformity of the stream produced by a venturi flume and with the possibility of employing this device for testing model seaplane floats. If the velocity of the stream is to be as high as 40 ft./sec., a net head (i.e., vertical distance between the supply level and the surface of the channel) of 25 ft. is required. In a venturi flume with an expansion ratio of 2, and working at this net head, the ratio of downstream to upstream depth would be at least 0.25. Hence the gross head, or vertical distance between the supply level and the bottom of the channel, would be 33 ft., and the depth of the issuing stream would be 8 ft., which is unnecessarily large. It will be appreciated that to attain velocities of this magnitude a very great expenditure of power is required, and therefore the cross section of the stream should be as small as possible consistent with the requirements of the experiments. The expansion ratio might be increased to reduce the depth of the stream and to raise slightly its velocity, but only at the expense of its uniformity. It is, however, possible that a satisfactory and more economical stream might be produced by means of a rectangular orifice inserted in the side of a large tank near the bottom, and discharging direct into an open horizontal channel of the same cross-section (Fig. 1). To avoid contractions, the orifice must be fitted with a trumpet.